An organic EL element utilizes electroluminescence of an organic material and is formed by laminating an organic charge transport layer and an organic luminescent layer between an anode and a cathode. Much attention has been paid to organic EL elements as high-intensity luminescence by low voltage direct current driving may be obtained therewith. Moreover, organic EL elements can be formed out of solids making them useful in flexible displays.
Luminescence properties such as luminescence brightness, luminescence efficiency, and luminescence uniformity of an organic EL element are often deteriorated after being driven for long periods of time. Causes of such deterioration include: oxidation of the electrodes due to penetration of oxygen into the organic EL element; oxidation decomposition of the organic material caused by heat generated during driving of the element; and oxidation of the electrodes and modification of the organic material caused by moisture in the air which penetrates into the element. Luminescent properties may also be deteriorated by peeling at an interface of the organic EL element. Peeling may be caused by oxygen and moisture, or by heat which generates stress due to differences in thermal expansion coefficients of the components of the element.
In order to reduce deterioration of luminescent properties, many technologies for sealing an organic EL element have been investigated in order to prevent the element from being contacted with oxygen and moisture. For example, Japanese Unexamined Patent Publication (Kokai) No. 9-148066 discloses a method comprising covering a pixel area formed by arranging an organic EL element as shown in FIG. 1. The element comprises transparent electrode 3, organic functional layer 4, metal cathode electrode 5 on substrate 1, with sealing cap 2 having a water-absorbing agent 6 affixed to the inner wall thereof. The interior of the element is filled with nitrogen gas, and sealing cap 2 is fixed to substrate 1 with adhesive 7. For another example, Japanese Unexamined Patent Publication (Kokai) No. 7-169567 discloses a method comprising using an oxygen-absorbing agent in place of the water-absorbing agent.
Various substances have been examined as water-absorbing agents. In particular, alkaline earth metal oxides such as barium oxide (BaO) and calcium oxide (CaO) have been widely investigated because the oxides differ from such water-absorbing agents that physically adsorb water as silica gel and zeolite in that the oxides can surely catch water molecules by chemical reactions, and because the oxides do not release the water molecules at high temperature. These oxides, however, are in the form of particles which must be affixed in a recess inside of the organic EL element which undesirably adds to the thickness of the element. Alkaline earth metal oxides can be applied to a so-called bottom emission type display apparatus from which display light is taken out of the substrate 1 side. However, alkaline earth metal oxides are not suitable for use in so-called top emission type display apparatus from which display light is taken out of the sealing cap 2 side opposite to the substrate 1. This is because they are opaque and thereby hinder emission of the display light. If oxides are to be used, they must be arranged so as not to cover the pixel area, and a new arrangement site must be provided.
Several proposals have been made in order to apply a water-absorbing agent to a top emission type display apparatus. For example, transparent, water absorbing polymers such as poly(vinyl alcohol) and nylon may be used, although these polymers physically adsorb water, which is not satisfactory as explained above. Japanese Unexamined Patent Publication (Kokai) No. 2001-357973 describes an organic EL element having a top emission structure and an arrangement of a particulate water-absorbing agent to such a degree that light transmission is not hindered. Japanese Unexamined Patent Publication (Kokai) No. 2002-56970 describes use of a plastic substrate in which a water-absorbing agent having a particle size smaller than the luminescence wavelength of the organic EL element. However, because inorganic particles are utilized as water-absorbing agents in both references, the water-absorbing agents are hardly arranged, and the primary particles thereof are hardly dispersed. As a result, light is scattered which undesirably lowers light transmission.
Japanese Unexamined Patent Publication (Kokai) No. 2003-142256 discloses use of a water-trapping film that absorbs little visible light and can be formed by coating a specific metal compound with a solvent. However, when this water-trapping film is applied to a flexible substrate, problems arise because the film is formed out of a low molecular weight compound, and thus is not flexible. Furthermore, in order to protect the element from both moisture and oxygen, respective trapping agents must be used. As a result, the problem that the arrangement sites of these agents are still more restricted arises.